149. The upper end of the humerus terminates in a circular head (fig. LXV. 9), which is received into the socket of the scapula (fig. LXV. 9. 7) termed, as has been stated (141), its glenoid cavity. The middle portion of the bone, or what is termed its shaft (fig. LXV. 8), diminishes considerably in magnitude, and becomes somewhat rounded (fig. LXV. 8), while its lower end again enlarges, and is spread out into a flattened surface of great extent (fig. LXIX. 1, 3, 2, 4). Of this broad flattened surface, the middle portion is grooved (fig. LXIX. 2): it is covered with cartilage; it forms the articulating surface by which the arm is connected with the fore-arm. On each side of this groove there is a projection of bone or tubercle, termed condyle (fig. LXIX. 3, 4), the inner (fig. LXIX. 3) being much larger than the outer (fig. LXIX. 4). The inner condyle gives origin to the muscles that bend, the outer to those that extend the fore-arm and the fingers (figs. LXXXIV. 1, 2, and LXXXV. 1). [Illustration: Fig. LXIX. 1. Lower extremity of the humerus; 2. grooved surface; 3. internal condyle; 4. external condyle; 5. the upper part of the ulna; 6. the head; 7. the neck; 8. the tubercle of the radius.] 150. The muscles that act upon the arm arise from the back (fig. LXXII. 2), the chest (fig. LXXI. 1), the clavicle (fig. LXXI. 1), and the scapula (fig. LXXI. 3); and they move the arm with freedom and power upwards, downwards, forwards, backwards, inwards, and outwards. The chief muscle that raises the arm is the deltoid (fig. LXXI. 3), which arises partly from the clavicle and partly from the scapula (fig. LXXI. 3). It has the appearance of three muscles proceeding in different directions, the different portions being separated by slight fissures (figs. LXXI. 3, and LXXII. 3). The fibres converging unite and form a powerful muscle which covers the joint of the humerus (fig. LXXI. 3). It is implanted by a short and strong tendon into the middle of the humerus (fig. LXXI. 4). Its manifest action is to pull the arm directly upwards. Its action is assisted by the muscles that cover the back of the scapula, which are in like manner inserted into the humerus, and which, at the same time that they elevate the arm, support it when raised. [Illustration: Fig. LXXI. View of the muscles on the fore part of the chest that act upon the arm. 1. The muscle called the great pectoral; 2. the small pectoral; 3. the deltoid; 4. the humerus.]